Practice question

Secretory pathway maintains SNARE availability through constant recycling machinery whose energy source ATP hydrolysis by NSF. After lipid merger SNARE proteins locked in four-helix bundle cis-complex embedded single target membrane deep energy well impossible separate spontaneously. To regenerate fusion-competent monomers ATP-driven chaperone required. Hexameric N-ethylmaleimide sensitive factor NSF 76 kDa per protomer double ring structure associates via adaptor alpha-SNAP decorating outside SNARE rod tetramer coating. Each SNAP C-terminus contacts one N-domain NSF hexamer forming 20S particle size sucrose gradient observed historically. ATP binding preorganizes NSF; cooperative hydrolysis D1 tier produces piston-like movement threading SNARE polypeptides through central tyrosine-containing pore loops unfolding superhelix releasing individual syntaxin SNAP-25 synaptobrevin. Freed syntaxin clamped by SM proteins, VAMP packaged into recycling vesicles. NSF does not function cargo receptor recognizing sorting signal, nor tether vesicles, nor phosphorylate Rab GTPases; specialized function SNARE chaperone powered by ATP. Depletion via N-ethylmaleimide or dominant negative E329Q accumulates cis complexes blocks secretion within minutes demonstrating essential housekeeping role supporting iterative fusion cycles.